This invention relates to an image fixing device for fixing an image formed on a sheet by a developing agent containing a thermoplastic material.
Japanese Pat. Publication Tokkai No. 58-88739 discloses a method of forming an image by using a photosensitive, pressure-sensitive recording (developing) material. According to this method, use is made of a photosensitive sheet with a light-transmissive sheet-like base material coated with microcapsules containing a radiation curable composition and a colorless dye and a transfer sheet coated with a developing agent having a chromogenic effect on this colorless dye and a thermoplastic pigment. If this photosensitive sheet is exposed to an image forming beam of light such as reflected light from an original document to be copied, those of the microcapsules (or the radiation curable composition inside) on the sheet exposed to light become hardened and a latent image corresponding to the original image is formed by the microcapsules which have not been hardened. If this photosensitive sheet and the aforementioned transfer sheet are thereafter superposed one on top of the other and pressed together, the microcapsules which have not been hardened rupture, allowing the colorless dye contained therein to flow out and to form a color image by contacting the developing agent on the transfer sheet.
In such a method for forming images, the chromogenic reaction of the colorless dye usually takes several hours under normal temperatures but can be completed instantaneously if temperature is raised to 60-100.degree. C. (Japanese Pat. Publication Tokkai No. 61-24495). It is also known that the thermoplastic pigment coating the transfer sheet becomes soft or molten when heated to 120-150.degree. C. although this depends somewhat on the kind of pigment and provides glossiness to the image by covering its surface (Japanese Pat. Publication Tokkai No. 60-259490). For this reason, it has been a common practice, after microcapsules are ruptured by pressing a photosensitive sheet and a transfer sheet together, to heat the transfer sheet by using a heater-fixer of a non-contact type such as an oven or a contact-type heater-fixer having a pair of heat and pressure rollers which are mutually opposite to each other in order to accelerate the chromogenic reaction or to provide glossiness to the image.
Non-contact type heater-fixers such as ovens, however, are poor in thermal conductivity efficiency and disadvantageous in that a large heat source is required. Moreover, they tend to cause the transfer sheet to curl up by the heat and hence may be unable to uniformly heat the transfer sheet. If a curled transfer sheet touches the heat source, there is the danger not only of smoking but also of a fire.
By contrast, a heater-fixer of a contact type which fixes an image by means of a pair of heat and pressure rollers are advantageous for having a high thermal conduction efficiency. Since the image is heated while the transfer sheet passes between rollers which are opposite to each other, however, there is the danger of the thermoplastic pigment becoming attached to the rollers to give the effect of offset printing. Although this phenomenon of offset printing can be prevented if the temperature is within a specified range, this temperature range is narrow and it is very difficult to control the heating temperature within such a narrow range. Moreover, a large amount of water, residual solvent and the like in the developing agent (colorless dye, radiation curable material, developing material and thermoplastic pigment) contained in the transfer sheet may be evaporated when heated and if the vapor thus generated is locally gathered between the transfer sheet and the heat roller, parts of the transfer sheet may not be heated sufficiently. If this happens and parts of the image are not heated sufficiently, the chromogenic reaction of the colorless dye becomes non-uniform and an image with color irregularities may be obtained. If the thermoplastic pigment cannot completely cover the surface of the chromogenic image, an image with irregularities in glossiness may result.